divyaan demonstrates practical approaches to enforcing Zero Trust and multi-tenancy in Kubernetes using custom admission webhooks on AKS, with code examples, Azure integrations, and actionable security policies.

Zero-Trust Enforcement and Multi-Tenancy Security in Kubernetes with Custom Admission Webhooks on AKS

Introduction

Securing Kubernetes clusters—especially on Azure Kubernetes Service (AKS)—requires applying Zero Trust principles and robust multi-tenancy controls. This article by divyaan walks through using custom admission controllers, OPA Gatekeeper, and Kyverno integrated with Azure services to automate policy enforcement and reduce risk.

Admission Controllers and Policy Engines

  • Admission Controllers in Kubernetes act as gatekeepers, intercepting Kubernetes API requests after authentication and before persistence. They enable validation/mutation of objects against security policies.
  • Strengthen admission with:
    • OPA Gatekeeper: Policy as code, tightly integrated with Azure Policy on AKS.
    • Kyverno: YAML-based policy engine.
    • Custom Admission Webhooks: Tailor policy logic, such as Zero Trust enforcement, to organizational needs.

Key Zero Trust Controls for Kubernetes

  • Least-Privilege Access: Use RBAC to grant only essential permissions, with individual ServiceAccounts per workload and no cluster-admin roles for apps.
  • Restrict Images to Trusted Registries: Block unverified public images using controllers or Azure Policy.
  • Deny Privileged Containers and Host Access: Prevent pods from running in privileged mode or mounting sensitive host paths (/etc, /var/run/docker.sock).
  • Default-Deny Network Policies: Set a deny-all default for ingress/egress traffic; allow only as needed.
  • Mutual TLS (mTLS): Use Istio or Linkerd service mesh for authenticated, encrypted internal traffic.
  • Continuous Audit and Drift Detection: Run OPA/Kyverno in audit mode to monitor ongoing policy compliance.
  • Enforce Runtime Security: Integrate Azure Defender for Kubernetes or Falco to detect anomalies and block privilege escalation.
  • Secure API Server Access: Lock down API with IP whitelisting, Azure AD integration, and RBAC.

Example: Custom Admission Controller for Pod Security

A sample Python Flask-based webhook enforces Zero Trust by requiring container images to come from a trusted Azure registry and all pods to include an environment label.

def validate():
    request_info = request.get_json()
    uid = request_info["request"]["uid"]
    pod = request_info["request"]["object"]
    violations = []

    # Only allow images from trusted registry
    trusted_registries = ["testtech.azurecr.io"]
    for container in pod.get("spec", {}).get("containers", []):
        image = container.get("image", "")
        if not any(image.startswith(reg) for reg in trusted_registries):
            violations.append(f"Image {image} not from trusted registry.")

    # Require 'environment' label
    labels = pod.get("metadata", {}).get("labels", {})
    if "environment" not in labels:
        violations.append("Pod missing required label: environment")

Steps to Implement

  1. Build and Deploy Webhook: Package the Flask webhook, mount TLS certs, expose via a ClusterIP service in AKS.
  2. Create TLS Certificates: Generate and store as a Kubernetes secret, include the service DNS for mutual trust.
  3. Register ValidatingWebhookConfiguration: Instruct K8s API to invoke your webhook on Pod creation/changes, ensuring secure admission.

Secure Multi-Tenant AKS Approaches

  • Namespace Isolation: Each tenant/team in its own namespace; set up RBAC and NetworkPolicies per-namespace.
  • Tenant-Specific RBAC: Use Azure AD groups integrated with K8s RBAC.
  • Network Fencing: Default-deny policies and Azure VNet peering/segmentation.
  • Resource Quotas: Set and enforce CPU/memory/storage limits per namespace.
  • Admission Controls: Use OPA Gatekeeper for namespace-specific guardrails.
  • Ingress/Egress Security: TLS for services and egress traffic controls per namespace.

Dynamic Enforcement in Webhooks

  • NetworkPolicy Enforcement: Webhook checks if both ingress and egress policies are enforced and do not allow open traffic.
  • ResourceQuota Validation: Webhook fetches current usage and denies resource requests exceeding quotas.

Best Practices

  • Secure Supply Chain: Use Trivy or Clair to scan container images in CI/CD; only allow signed/trusted images in AKS.
  • Runtime Threat Detection: Tools like Falco can trigger alerts on suspicious activity.
  • Observability: Integrate Prometheus/Grafana for metrics, central log aggregation (Elasticsearch), and Microsoft Sentinel for threat detection.
  • Incident Response: Define and test IR playbooks, keep regular backups, and practice secret rotation.

Conclusion

By combining OPA Gatekeeper, Kyverno, Azure integrations, and custom admission controllers, you shift Kubernetes security from passive monitoring to proactive enforcement. Zero Trust and multi-tenant controls ensure workloads in AKS remain compliant, isolated, and resilient against threats.

For code samples and deployment manifests, see Kubernetes Custom Admission Controller.

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